The Controller Area Network (CAN) bus format was initially developed to permit a number of separate electronic devices in a vehicle such as an automobile to communicate over a single data bus. Data is transmitted over CAN bus among the various devices, each connected to a node on the bus, with each data message having an identifier which specifies the device to which the message is directed. Since the introduction of the CAN bus in the 1980's, its use has spread beyond automobiles to numerous machine and industrial applications, and a number of variations of the CAN bus format have been adopted. In all of these variations, however, data is transmitted over a single terminated twisted pair cable at a high frequency, on the order of 250 Kbit/sec (IS011519 or SAE J1939) and up to 1 Mbit/sec (IS011898). The CAN bus format contemplates a differential signal on the twisted cable pair, with the data signal on one line of the twisted cable pair being a coincident mirror image of the data signal on the other line. In view of the relatively high frequency at which data messages are transmitted, it is necessary to terminate the twisted cable pair at each of its ends with an appropriate impedance to reduce noise and signal reflections that would otherwise degrade system operation.
Because of the flexibility of the CAN bus format, it has become common to use CAN bus extensions. CAN bus extensions can plug into the ends of the main CAN bus of a machine when additional devices are added either temporarily or permanently. When an extension is added, of course, the node which was formerly the end node of the CAN bus becomes an intermediate node. A terminating impedance which was formerly across the twisted cable pair at that node must then be removed, and a proper terminating impedance provided at the end of the CAN bus extension. In the past, terminating impedances were plugged in at the ends of the CAN bus or CAN bus extensions, as needed. Alternatively, terminating impedances were provided at the ends of the CAN bus and extensions, and manually switched across the twisted pair cable when needed. The difficulty with these termination approaches is that they rely on the user remembering to terminate the CAN bus properly. While several automated termination arrangements have been developed, these have tended to be complicated and expensive.